Epoxides are highly versatile synthesis units in organic and macromolecular chemistry. The high ring strain of epoxides (>20 kcal/mol) is responsible for their good reactivity toward nucleophiles (ring opening) and thus provides a route to a series of interesting structures. In addition to amines, mercaptans and alcohols, carboxylic acids in particular find use as nucleophiles since this substance class is highly notable for being conveniently obtainable. However this very often requires the use of catalysts since carboxylic acids exhibit a lower nucleophilicity than, for example, the corresponding amines.
In addition to monomeric epoxides such as glycidyl acrylate or butyl glycidyl ether for example, polymeric epoxides such as inter alia polyglycidyl compounds of the bisphenol A type, epoxy-functional polymers of vinyl monomers but also epoxysiloxanes are indispensable to material science and industry.
Polysiloxanes comprising acrylic ester groups (acrylate groups) have hitherto proven themselves as high-energy-radiation-curable additives, for example for printing inks and for producing paint binders or for coatings for plastic, paper, wood and metal surfaces. Curing is preferably effected at low temperatures and induced by electron radiation or by UV radiation in the presence of prior art photoinitiators, for example benzophenone and derivatives thereof (EP 1897917).
A series of catalysts are suitable for epoxide ring opening with acids. A very wide variety of catalysts for the reaction of glycidyl compounds with carboxylic acids are thus known from coatings technology. This reaction is often used as a cross-linking reaction for curing paints for example. In addition to ammonium salts and phosphonium salts and amines and phosphines, certain metal compounds are also described. These catalysts known in the prior art often prove unselective and thus disadvantageous since they have the potential to break bonds in silicone chains for example and thus to bring about undesired side reactions such as molar mass decrease or skeletal rearrangement. These catalysts can moreover also trigger undesired side reactions at the silicone-bonded organic groups such as for instance homopolymerization of epoxy groups, conversion of carboxylic acids into amides or Michael additions of amines onto acrylate groups or else free-radical homopolymerization of acrylates, which can be triggered by a redox reaction with a great many metal compounds.
Mol Divers (2013) 17; 9-18 describes the use of ionic liquids such as butylmethylimidazolium bromide as catalyst for ring opening of epoxides with aliphatic and aromatic acids. This catalyst releases the strong acid HBr during the reaction so that acid-labile polymers such as siloxanes cannot be converted.
EP 1693359 describes catalysis with certain weakly Lewis-acidic borane compounds such as trisdimethylaminoborane. A flash point of −20° C. impedes handling so that the reaction is carried out at 70° C. leading to lengthy reaction times.
WO 0177240 describes the use of chromium salts for ring opening of epoxidized siloxanes with acrylic acid and subsequent use of these materials in UV-curable release coatings. Various solvents are used here which are said to ensure improved commixing of the catalyst with the epoxysiloxane.